1. Field of the Invention
The present invention relates to an optical element and an optical add-drop module using such a filter, and more particularly, to an optical element capable of selectively transmitting or reflecting light having a specific wavelength while optionally changing the wavelength itself, and an optical add-drop module which has modularized these devices.
2. Discussion of the Related Art
Optical bandpass filters have been used for selecting light with a specific wavelength from inputted light rays. Among the bandpass filers, a tunable optical filter capable of changing a wavelength to be selected is applied various cases. For example, the tunable filter is used as a component of a multi-channel analyzer so as to change the wavelength to measure properties of a substance. The typical application has been proposed in JP-A 08-227014 (1996). This tunable optical filter has realized tunable characteristic by changing the physical film thickness.
FIG. 1 shows a tunable optical filter in accordance with a first prior art. This tunable optical filter 101 has a structure in that a dielectric multilayer film filter 102 is attached onto a movable stage 103, and maintained thereon in a manner to freely shift in the direction of arrow X. The dielectric multilayer film filter 102 is constituted by dielectric multilayer films 109 formed by alternately stacking a number of first and second dielectric films 107 and 108 having mutually different refractive indexes with continuously varied thicknesses on one surface of a transparent substrate 106. The filter 102 is attached with its length direction being set in the direction of arrow X. Here, FIG. 1 indicates these stacked layers in a schematic manner. The central wavelength of the filter 102 is proportional to the optical film thickness nL. Here, the value n denotes the effective refractive index of a medium which constitutes the filter and the value L denotes its film thickness. By forming the dielectric multilayer film filter 102 having bandpass filter characteristics, with the film thickness having a gradient along a one-dimensional direction (the direction of arrow X), it is possible that in the place where the film is thin, a transmission peak is set at a short wavelength, and that in the place where the film is thick, the transmission peak is set at a long wavelength.
Upon using this tunable optical filter 101, a photodiode (PD) 111 is secured at a fixed position so that incident light 112 is made incident on the photodiode 111 through the dielectric multilayer film filter 102. In this layout, when the filter 102 is shifted in the direction of arrow X, the peak transmission wavelength is changed in response to the shift in the direction of arrow X, so that the wavelength of receiving light of the photodiode 111 changes continuously.
FIG. 2 shows the relationship between the selected wavelength when the tunable optical filter of is shifted in the direction of arrow X in FIG. 1 and transmissibility of the light. As shown in FIG. 2, when the tunable optical filter is, for example, shifted by the movable stage 103 from a first position X1 to a second position X2 in the direction of arrow X, the transmitting wavelength of light changes continuously from a first wavelength λ1 to a second wavelength λ2 in accordance with the shift. In FIG. 2, for example, two wavelengths are shown between the first position X1 and the second position X2.
In conventional tunable filters, only the bandpass filter referred to as a single cavity type has been put into practical use. This is because, as the number of cavities increases to double, triple and so on, the film thickness control of each layer which constitutes the multilayer film filter becomes difficult. In general, in the tunable filter device used in the optical communication technique, those filter devices having a substrate dimension of about 50×4 mm are used. In the first conventional example, it is difficult to form a dielectric multilayer filter having 100 or more layers over the entire surface of a large substrate with a film thickness gradient ranging over 50 mm in a one-dimensional direction, with each of desired film-thickness distributions. Moreover, it is difficult to produce the filters with high yield, causing very high costs. In order to realize a tunable bandpass filter having a narrow band of 1 nm or less in the transmission band width in the optical communication field, it is required that films should be formed with a desired film thickness of each layer in an error level of 0.01% or less. The range of the conforming article which satisfies this accuracy is generally almost only the 3 to 10 mm range, due to film thickness distribution variations upon film formation in a film forming device, aging variations of the film formation condition and the like. Therefore, tunable bandpass filters obtained through another different technique have been desired.
The optical communication system obtained by the wavelength division multiplexing (WDM) system has been noticed as the next generation large-capacity optical communication system. In the optical communication system of the WDM system, optical signals (λ1, λ2, . . . , λn) which have respectively different wavelengths are allowed to propagate through one optical fiber. Therefore, each of nodes needs to have the ability of carrying out a process for adding (inserting) or dropping (picking out) optical signals with a single or a plurality of wavelengths on demand. An optical add-drop module (OADM) is used to meet the demand. The OADM is an optical separation/coupling device that is inserted into a network of the optical communication, and extracts information from the high-speed communication path and introduces information into the communication path.
In the conventional OADM, the wavelength to be added or dropped is fixed to a single wavelength. In other words, only the optical signal of specific one wavelength corresponding to a specific signal channel is extracted (dropped) from a multi-wavelength signal component that propagates through a single optical fiber, and an optical signal having the same wavelength is introduced (added) thereto. Moreover, in order to allow further flexible signal processing, an OADM that can dynamically select an optional signal channel, and add or drop an optical signal of the corresponding wavelength is required. In such a module, not a bandpass filter for allowing only one of the wavelengths to pass, but a bandpass filter with tunable characteristic is required.
In the tunable optical filter 101 shown in FIG. 1, the selection of wavelength is carried out by mechanically shifting the movable stage 103. Therefore, even in an attempt to apply the tunable optical filter 101 to this OADM, there is a limitation in the response speed upon changing the wavelength to be added or dropped. For this reason, although it is possible to meet a limited request such as a request for assigning a specific communication path to a specific customer during a predetermined time zone, it is impossible to dynamically select a wavelength to be added or dropped at a high speed.
Researches for tunable optical filters with higher response speed have been made. JP-A 11-119186 (1999) has proposed a tunable optical filter in which a wavelength to be selected is made variable by changing the refractive index of liquid crystal under control of an applied voltage. FIG. 3 shows a structure of the tunable optical filter in accordance with a second prior art. The tunable optical filter 121 is provided with a first layer 122, a second layer 123 that is placed on the light-incident side with respect to the first layer 122, and a third layer 124 that is placed on the light emission side with respect to the first layer 122. Optical mirrors (dielectric mirror) films 125A and 125B are placed on the second and third layers 123 and 124 on the sides closer to the first layer 122. Moreover, the other layers except for these layers on the second and third layers 123 and 124 constitute transparent electrodes 126A and 126B. Transparent substrates 127A and 127B are formed on the outside of the second and third layers 123 and 124, and anti-reflection coat films 128A and 128B are formed on the outside of these layers.
The first layer 122 of the tunable optical filter 121 is made of a material the refractive index of which changes in response to an electric field. This material is formed by dispersing liquid crystal droplets having a diameter of not more than 150 nm in a light-transmitting medium such as polymer (high molecules) and quartz glass. The second layer 123 is transparent within a wavelength band to be used, and has no polarization dependency with respect to changes in the refractive index with or without the application of a voltage. For this reason, no polarization dependency occurs in the characteristic of the tunable optical filter 121. With respect to the matrix medium in which liquid crystal droplets are dispersed, polymers having a superior light-transmitting property without any optical anisotropy are used; and examples thereof include PMMA-based polymers, polystyrene-based polymers, polycarbonate-based polymers, thermo-setting or photo-curing acryl-based polymers, epoxy-based polymers, polyurethane-based polymers, polyisocyanate-based polymers, polyene polythiol-based polymers, glass and the like.
The tunable optical filter 121 is a Fabry-Perot etalon-type tunable filter that uses a polymer as the matrix medium with a micro-liquid crystal droplet dispersion polymer being sandwiched between cavities thereof. In the case where no voltage is applied to the transparent electrodes 126A and 126B, the liquid crystal molecules in the liquid crystal droplet in the first layer 122 are arranged in random directions. Therefore, the refractive index that is exerted on light made incident on the micro-liquid crystal droplet dispersion polymer layer corresponds to an averaged refractive index of refractive indexes (ne, no) of nematic liquid crystal. When a voltage is applied by using an AC power supply (not shown), the liquid crystal molecules are aligned in the direction of the applied electric field, and the refractive index approaches no. In response to such a change in the refractive index, the transmission spectrum to incident light 131 made incident on the tunable optical filter 121 is changed, with the result that the wavelength of the transmission light 132 is changed.
Here, the tunable optical filter 121 shown in FIG. 3 requires a high voltage of not less than 100 volts as an applied voltage. Moreover, many transmission peaks of a wavelength occur cyclically with respect to the wavelength axis. Consequently, in order to shield transmission peaks occurring outside the originally required wavelength band, another filter is further required. Moreover, the filter properties are more susceptible to environmental influences such as humidity and temperature. For this reason, it becomes difficult to obtain superior properties with respect to incident light with a high output.
Moreover, in the tunable optical filter 121 shown in FIG. 3, the driving operation for liquid crystal needs to be carried out by an AC current, and is not carried out by a DC current. An attempt to carry out a DC driving operation would cause a dielectric breakdown. The resulting problem is that, although the tunable optical filter 121 is allowed to change the wavelength, it fails to maintain the wavelength in a fixed state within a desired filter wavelength.
The foregoing description has discussed conventional problems with tunable optical filters, and with respect to optical elements other than tunable optical filers that selectively absorb and reflect a specific wavelength component also, conventionally, the same problems have been raised. Moreover, with respect to optical add-drop modules which add and drop optical signals by using tunable optical filters also, conventionally, the same problems have been raised.